[LAD] Is -ffast-math safe for audio?

Robin Gareus robin at gareus.org
Fri Nov 23 14:09:02 CET 2018

On 11/23/2018 01:00 PM, Will Godfrey wrote:
> Thanks for going into this in such detail Robin. I never realised fp stuff
> could be *quite* so, umm, approximate!

Depending on context and the maths, the difference may not matter at
all, or may be off completely..

  float a = (1 + 1e20) - 1e20;
  float b = 1 + 1e20; b -= 1e20;
  printf ("%f %f\n", a, b);

Any guesses what this code prints without compiling it? :)

> I was using O3. Following on from this I found that if I removed either O3 or
> ffast-math the problem disappeared.

OK, so SSE/AVX/compiler-intrinsics are the issue at hand.

> It's quite possible that I'm over-thinking this as there are actually only a
> few iterations before the initial figures are re-calculated, but I don't like
> mysteries :(

You could also disable this for a specific function only

void different_mysteries_here (float val)


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